Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a series of bars and spaces of varying widths, the bars and spaces having differing light reflecting characteristics. Some of the more popular bar code symbologies include: Uniform Product Code (UPC), typically used in retail stores sales; Code 39, primarily used in inventory tracking; and Postnet, which is used for encoding zip codes for U.S. mail. Bar codes may be one dimensional (1D), i.e., a single row of graphical indicia such as a UPC bar code or two dimensional (2D), i.e., multiple rows of graphical indicia comprising a single bar code.
Systems that read bar codes (bar code readers) electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. The characters are then typically represented in digital form and utilized as an input to a data processing system for various end-user applications such as point-of-sale processing, inventory control and the like.
Bar code readers that read and decode bar codes employing imaging systems are typically referred to as imaging-based bar code readers or bar code scanners. Imaging systems include charge coupled device (CCD) arrays, complementary metal oxide semiconductor (CMOS) arrays, or other imaging pixel arrays having a plurality of photosensitive elements (photosensors) or pixels. An illumination apparatus or system comprising light emitting diodes (LEDs) or other light source directs illumination toward a target object, e.g., a target bar code. Light reflected from the target bar code is focused through a system of one or more lens of the imaging system onto the pixel array. Thus, the target bar code within a field of view (FV) of the imaging lens system is focused on the pixel array.
Periodically, the pixels of the array are sequentially read out generating an analog signal representative of a captured image frame. The analog signal is amplified by a gain factor and the amplified analog signal is digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals representative of the captured image frame and attempts to decode the imaged bar code.
As mentioned above, imaging-based bar code readers typically employ an illumination apparatus to flood a target object with illumination from a light source such as a light emitting diode (LED) in the reader. Light from the light source or LED is reflected from the target object. The reflected light is then focused through the imaging lens system onto the pixel array, the target object being within a field of view of the imaging lens system.
The illumination system is designed to direct a pattern of illumination toward a target object such that the illumination pattern approximately matches the field of view (FV) of the imaging system. In some of the illumination systems, however, illumination intensity of the illumination pattern may not be very uniform, especially near the edges of the illumination pattern. It is desirable to have an illumination pattern that has substantially uniform illumination intensity within the entire illumination pattern. It is also desirable to have an illumination pattern with sharply defined edges.
Accordingly, there is a need for improved methods and apparatus for defining the illumination field of view of the barcode readers.